An amplifier is typically used to amplify signals such as, for example, high frequency radio frequency (“RF”) or microwave signals. Amplifiers may be multi-stage devices, where the input signal to be amplified passes through a series of amplification stages each comprising one or more relatively small amplifiers. Generally, each stage of the multi-stage amplifier steps up the amplification of the signal, such that the output signal of the amplifier has been appropriately amplified.
By way of example, in high-power amplifiers (“HAP's”), increasing the RF gain (i.e., the RF signal amplification) typically requires a number of amplification stages. Each amplification stage is typically powered by bias circuitry that provides a bias voltage and current to each stage. Processing variations, from lot to lot in the manufacture of amplifiers, may cause variations in the bias voltage and current provided to the amplification stages. In some circuits, small changes in the process of constructing the die at a wafer fabrication plant can result in drain current variation as between two die that are biased identically. Furthermore, changing the temperature of bias circuitry may cause the bias circuitry to bias an amplifier differently at different temperature levels.
Therefore, active bias circuits are used to at least partially compensate for these variations and to control the bias voltage substantially independent of these changes. Controlling the bias in this manner is known as active biasing.
Although active biasing is desirable, active biasing circuitry typically occupies a relatively large amount of space and increases the costs associated with building an amplifier circuit. Therefore, conventional wisdom has been that when active bias is provided, a single active bias circuit is used to provide a common active bias to all of the stages of the multi-stage amplifier. It is generally understood to be cost and space prohibitive to provide a separate active bias circuit for each amplifier stage.
Thus, typically, all of the amplification stages are actively biased together. In other words, the current to the whole die (i.e., all the amplification stages) is controlled by one active bias circuit. However, when a common bias is provided to all of the stages and a compression sweep is run on a multi-stage amplifier at least some of the stages may experience a certain amount of RF gain expansion and/or compression. Typically, gain expansion and compression may cause the bias current to increase. The bias current to each stage tends to increase with an increase the RF signal power at the input of that stage. This phenomenon is referred to as “current pump-up.” The last stage the multi-stage amplifier output, is usually the first to show signs of this current pump-up, and the first stage is usually the last to show signs of current pump-up. In some cases, active biasing the whole die may compensate for current pump-up, but may also limit the output power of the HPA.
In addition, active bias circuits may suffer from slow turn on time and active bias oscillations. Thus, a need exists for an improved system and method for the active biasing of multi-stage amplifiers and/or for reducing the limitation on output power of the multi-stage amplifiers. Furthermore, a need also exists for active biasing that improves settling time and turn on time and improves circuit stability.